Method for detecting geopressures

ABSTRACT

Interstitial waters of geopressured shale sections and the transition zone lying above the geopressured shale zones are measured for total salinity or elemental cationic concentration. Variances in these measurements are utilized to obtain a real time method for detecting geopressured shale zones.

United States Patent 1191 1111 3,770,378

Russell 2 i 1 Nov. 6, 1973 [5 METHOD FOR DETECTING 3,628,131 12/1971Overton 23 230 EP x GEOPRESSURES [75] Inventor: Kenneth L. Russell,Tulsa, Okla.

. Primary Examiner-Robert M. Reese [73] Assignee: Cities Service OilCompany, Tulsa, J Reine" Okla.

[22] Filed: June 18, 1971 [21] Appl. No.: 154,622

57 ABSTRACT 52 us. c1 23/230 EP, 73/153 175/50,

324/1 Interstitial waters of geopressured shale sect ons and 51 Im. c1.E2lb 47/00, GOln 33/24 the transition Zone lying above the geopressuredShale [58] Field of Search 23/230 EP; 175/50, Zones are measured forsalinity of elemental 175/58, 60, 324/1; 73/151, 153 ionicconcentration. Variances in these measurements are utilized to obtain areal time method for detecting [56] References Cited geopressured shalezones.

UNITED STATES PATENTS 7 3,237,094 2/1966 Blackborn et al. 324/1 6Claims, N0 Drawings METHOD FOR DETECTING GEOPRESSURES BACKGROUND OF THEINVENTION The present invention relates to a method for determining thepresence of geopressured subterranean shale formations. Moreparticularly, the present invention discloses a method for the chemicaldetermination of the salinity or ionic concentration of drill bitcuttings in order to detect the presence of a geopressured shaleformation.

In applying technology to a drilling operation, it is a necessarycriteria that the presence and lithology of formations encountered by adrill bit be recognized. Many methods have been presented for thedetermination of geopressured shales encountered during a drillingoperation in orderto provide an alarm system for detecting when a drillbit enters these hazardous formations. One example of such a method istaught by Jordon, et al. U. S. Pat. No. 3,368,400, METHOD FORDETERMINING TOP OF ABNORMAL FORMA- TION PRESSURES, which teaches aprocess for detecting when a borehole enters a geopressured shalesection through utilization of the penetration rate of the drill bit asa measured variable. Through the teachings of Jordon, one determines thepenetration rate of the formations being encountered by the drill bitand by logging these penetration rates determines the exact location anddepth of geopressured shale sections so that mud weights and drillingvariables may be changed to anticipate .well blowouts. y

In changing formations, some well control may be provided by offsettingwells, although exact alignment of lithology is difficult to achieve aslithology data is often not availablefor the areas being encountered bythe drill bit. Well logging by conventional downhole wellbore sondes isparticularly difficult in the drilling of wells as a trip must be madewith a drill string before the logging sonde may be lowered into theformation. Forward looking lithology gathering devices are generallyinapplicable as a seismic tool is required which will impart energy tothe earth and allow geophones located within a logging. sonde, locatedat bottomhole, to receive the reflected seismic waves encountered fromlower formations which the drill bit has not yet entered. These methodsare particularly hazardous, expensive and time consuming. Forwardlooking devices are not presently practiced on conventional drillingrigs due to these limitations.

What is required is a method for determining the lithology and thepresence of geopressured-formations during drilling from drill bitcuttings.

It is an object of the present invention to provide a method fordetermining the presence and lithology of geopressured shale formationsencountered by a drill bit.

It is a further object of the present invention to provide a method forthe chemical determination of the salinity or elemental cationicconcentration of drill bit cuttings in order to determine the presenceof geopressured shale formations.

It is still a further object of the present invention to provide amethod for the analysis of the interstitial water contained withindrillbit cuttings in order to determine the salinity or elementalcationic concentration thereof for the detection of geopressured shalezones.

With these and other objects in mind, the present invention may be morefully understood through the following discussion and description.

SUMMARY OF THE INVENTION The objects of the present invention areaccomplished through a method for the detection of geopressured shaleformations during drilling of a well which comprises either measuringthesalinity or the cationic concentration of one or more elements in thedrill bit cuttings as they are produced from the wellbore. A significantchange in salinity or cationic concentration indicates the entrance ofthe drill bit into a geopressured shale formation. Generally, thecationic concentration or salinity of the drill bit cuttings is measuredby pressing the interstitial waters from the drill bit cuttings andanalyzing the interstitial waters for total salinity or elementalcationic concentration. It is preferred that the cationic concentrationbe measured for one or more elements selected from the group consistingof sodium, calcium, potassium, iron and magnesium. The salinityconcentration in waters may be analyzed by measuring the resistivity ofthe waters, with the resistivity of the waters versus depth beingplotted in order to form a resistivity log to aid in the determinationof any change in salinity.- Similarly, the cationic concentration may beplotted versus depth to form a log to aidin the determination of anychange in the cationic concentrations.

DETAILED DESCRIPTION OF THE INVENTION The method of the presentinvention involves the extraction of interstitial waters from shalecuttings at the drill site and measurement of the salinity or cationicconcentration thereof to obtain a reliable real time method fordetecting geopressured shale formations. It has been found that theinterstitial waters of geopressured shale sections, and in particular,the overlying 20-100 foot transition zone associated with thegeopressured shale zone, are less saline or fresher than theinterstitial waters of the other overlying formations. The salinitydifference found is generally a two-fold factor, such that it is readilymeasured with small errors in analysis being relatively insignificant.In general, it is found that the formation below a pressure cap or'shaleis highly reduced chemically, the cap being a phase boundary. Highreactive material is found, whose depth varies with temperature. Theshales may be very conductive in this hydrocarbon concentratingenvironment. Simultaneous measurement of the resistivity of drill bitcuttings form electrical logs of resistivity which are indicative ofgeopressured shale zones. In general, one may measure the total salinityof the drill bit cuttings or the cationic concentration of one or moreelements selected from the group consisting of sodium, calcium,potassium, iron, and magnesium in order to determine the presence of thesand-shale boundaries. For example, the sodium cationic concentration orpotential may be measured to yield geological, as well as drillinginformation. In general, sodium ionic concentration is from about 5 toabout 10 times that in the shale in the sand zone overlying ageopressured shale formation. Therefore, a rapid decrease in sodiumionic concentration indicates a geopressured shale is being penetratedby the drill bit. Potassium content is generally greater in shales,while calcium content exhibits a similar concentration as that ofsodium. Various and sundry other elemental cationic concentrations maybe measured, including iron and magnesium.

As the salinity differs as one crosses a sand-shale boundary, anelectric log may be made from drill bit cuttings. For example, thesodium cationic concentration or potential of the waters encounteredgenerally vary in the same manner as an SP log later taken in thewellbore. The shale resistivity performs similar to the wire lineresistivity log.

The following procedure may be utilized for measuring cationicconcentration or total salinity resistivity in interstitial waters ofshales and overlying formations. In general, one monitors the drillingvariables such as rate of penetration, to deduce possible lithologiesand the presence of geopressured shale formations. This procedureeliminates the requirement of a continued chemical analysis as isutilized in the present method for the entire drilling sequence. Onerecords the time when potential samples will arrive at the surface, ifrequired, drilling may be suspended until samples arrive. The samplesare gathered at the shale shaker and washed on a screen having about ii-inch mesh, utilizing fresh or distilled water with the mud and finecuttings being discarded. Large drill bit cuttings are gathered by handor mechanically in order to provide fresh pieces of shale which are freeof mud. These shale cuttings or drill bit cuttings are individuallydried with paper towels. The dried samples may then be further dried atroom temperature until no moisture is visible. The room drying generallyrequires minutes. A press may be utilized and loaded with 40 to 50 shalechips, which represents approximately 15 grams of sample, so as topressure the shale chips to pressures in excess of 5,000 psi. Theinterstitial waters are driven from the drill bit cuttings duringpressuring so that from about 0.5 to about 1.0 milliliters of water maybe provided for analysis from the 40 to 50 grams of shale chips. Thewater is analyzed for whichever elemental cationic concentration isdesired or for the total salinity of the interstitial waters. Generally,the sodium cationic concentration and the total salinity may becorrelated very closely as approximately 90 percent of the salts in theinterstitial waters are sodium salts. A 50 percent decrease in sodiumcationic concentration marks the beginning of an entrance of the drillbit into a geopressured zone. The elemental cationic concentrations orthe total salinity-resistivity may be plotted in order to provide a logto accurately determine the transition and change from the sandshaleboundary into the geopressured shale zone in order to rapidly determinethe presence of geopressured shales.

The process disclosed herein is applicable as a system for thedetermination of geopressured shales as utilized in conjunction withmechanical indications in order to verify a mechanical determination ofoverpressured formations. The system approach avoids confusion of thegeopressured shales with silty shales or fault zones. The systems formof determination is exclusive for geopressured shales so as to provide afailsafe technique.

While the invention has been described above with respect to certainembodiments thereof, it will be understood by those skilled in the artthat various changes and modifications may be made without departingfrom the spirit and scope of the invention as set forth herein.

Therefore, 1 claim:

1. A method for the detection of geopressured shale formations duringthe drilling of a well, which comprises:

a. removing interstitial waters from drill bit cuttings as they areproduced from the well bore,

b. measuring the cationic concentration of one or more elements in theinterstitial waters; and

c. detennining a large change in cationic concentration which indicatesthe entrance of the drill bit into a geopressured shale formation.

2. The method of claim 1 in which the interstitial waters are pressedfrom the drill bit cuttings.

3.The method of claim 1 further comprising plotting the cationicconcentration of the waters versus depth to form a log to aid in thedetermination of any change in the cationic concentration.

4. A method of claim 3 in which the cationic concentration measured isfor one or more elements elected from the group consisting of sodium,calcium, potassium, iron and magnesium.

5. A method for the detection of geopressured shale formation during thedrilling of a well which comprises:

a. producing drill bit cuttings having interstitial water from saidwell;

b. pressing said interstitial waters from said drill bit cuttings;

c. measuring the resistivity of said interstitial waters to analyze thesalinity of said waters; and

d. determining a large decrease in said salinity which indicates theentrance of the drill bit into a geopressured shale formation.

6. The method of claim 5 further comprising plotting the resistivity ofthe waters versus depth to form a resistivity log to aid in thedetermination of any decrease in salinity.

2. The method of claim 1 in which the interstitial waters are pressedfrom the drill bit cuttings.
 3. The method of claim 1 further comprisingplotting the cationic concentration of the waters versus depth to form alog to aid in the determination of any change in the cationicconcentration.
 4. A method of claim 3 in which the cationicconcentration measured is for one or more elements elected from thegroup consisting of sodium, calcium, potassium, iron and magnesium.
 5. Amethod for the detection of geopressured sHale formation during thedrilling of a well which comprises: a. producing drill bit cuttingshaving interstitial water from said well; b. pressing said interstitialwaters from said drill bit cuttings; c. measuring the resistivity ofsaid interstitial waters to analyze the salinity of said waters; and d.determining a large decrease in said salinity which indicates theentrance of the drill bit into a geopressured shale formation.
 6. Themethod of claim 5 further comprising plotting the resistivity of thewaters versus depth to form a resistivity log to aid in thedetermination of any decrease in salinity.